3GPP LTE (3rd Generation Partnership Project Long Term Evolution) employs SC-FDMA (Single Carrier-Frequency Division Multiple Access) as its uplink access scheme. SC-FDMA features a low PAPR (Peak to Average Power Ratio) achieved by the single-carrier scheme of SC-FDMA, flexible data assignment to sub-carrier frequencies, resilience to multipath in frequency-domain signal processing on the receiving side, and the like.
In SC-FDMA, on the transmitting side, SC-FDMA symbols are generated, for example, by converting time-domain symbols into frequency components by a DFT (Discrete Fourier Transform), mapping the frequency components to different sub-carriers, then, converting the mapped frequency components back into a time-domain waveform by an IDFT (Inverse Discrete Fourier Transform), and adding a CP (Cyclic Prefix) to the time-domain signal. Correspondingly, on the receiving side, the time-domain signal is recovered by converting the time-domain signal from the transmitting side into frequency components by a DFT, performing frequency equalization processing on the frequency components, and performing an IDFT on the signal after the frequency equalization processing. As described above, in SC-FDMA, the DFT on the transmitting side (hereinafter referred to as a transmitting DFT) corresponds to the IDFT on the receiving side (hereinafter referred to as a receiving IDFT), and the IDFT on the transmitting side (hereinafter referred to as a transmitting IDFT) corresponds to the DFT on the receiving side (hereinafter referred to as a receiving DFT).
As a new control scheme for the code rate of turbo coding, puncturing in the frequency domain (frequency puncturing, which may hereinafter be abbreviated as FP) has been drawing a lot of attention (see Non-Patent Literature (hereinafter, as abbreviated NPL) 1, for example). The frequency puncturing is a puncturing scheme which is basically used in SC-FDMA systems, and in which puncturing is performed on frequency-domain signals after the transmitting DFT.
A comparison is made below between the frequency puncturing and puncturing in the time domain (time puncturing, which may hereinafter be abbreviated as TP), which is the control scheme for the code rate of turbo coding according to the related art (see FIGS. 1A and 1B).
In the time puncturing, puncturing is performed on encoded bits in the time-domain immediately after turbo coding (i.e., before the transmitting DFT) on a per bit basis. For example, in FIG. 1A, the last two bits are punctured (thinned) among eight encoded bits. In contrast, in the frequency puncturing, puncturing is performed on data to be punctured, in which the encoded bits are convoluted into a plurality of symbols in the frequency domain, on a per symbol basis. For example, in FIG. 1B, the two symbols that are highest in frequency are punctured (thinned) among eight symbols mapped to eight sub-carriers. Thus, in the frequency puncturing, some components of each encoded bit are punctured to a similar extent, while in the time puncturing, the whole of some encoded bits are completely punctured. In other words, contrary to the time puncturing, in the frequency puncturing, the whole of some encoded bits are not completely punctured. In consequence, assuming that transmission power is the same (i.e., the power corresponding to the punctured components is the same), the frequency puncturing allows for more parity bits per transmission than the time puncturing. For example, in the frequency puncturing shown in FIG. 1B, although some components of each encoded bit are punctured, all the eight encoded bits are transmitted, while in the time puncturing shown in FIG. 1A, two bits are completely punctured, and six encoded bits are transmitted. The frequency puncturing may therefore improve an error correction coding gain as compared to the time puncturing by an increase of the number of transmitted parity bits.
In the frequency puncturing, some frequency components of each encoded bit are punctured between the transmitting DFT and the receiving IDFT (i.e., after the transmitting DFT and before the receiving IDFT). This deteriorates unitarity (orthogonality) between the DFT matrix used in the transmitting DFT and the IDFT matrix used in the receiving IDFT, resulting in inter-symbol interference. In contrast, in the time puncturing, some encoded bits are punctured before the transmitting DFT. Thus, the unitarity is retained between the DFT matrix used in the transmitting DFT and the IDFT matrix used in the receiving IDFT.
As described above, in the frequency puncturing and the time puncturing, there is a trade-off between “improving the error correction coding gain by an increase of the number of parity bits” and “retaining the unitarity between the DFT matrix on the transmitting side and the IDFT matrix on the receiving side.”
HARQ (Hybrid Automatic Repeat reQuest) is used as an error control technique in packet transmission. HARQ is a technique that combines ARQ (Automatic Repeat reQuest) and error correction coding. 3GPP LTE employs, as HARQ, IR (Incremental Redundancy) using a CB (Circular Buffer). In such IR using a CB, among systematic bits (information data itself) and parity bits (redundant bits) constituting encoded data after turbo coding, the systematic bits are transmitted first, and when an error occurs in the received packets on the receiving side, the parity bits are retransmitted. Since the larger number of parity bits can lead to a higher error correction coding gain, the number of retransmissions can be reduced by an increase in the number of parity bits per transmission (or retransmission).
An example of transmission schemes with HARQ (IR using a CB) used in 3GPP LTE will be described in reference to FIG. 2. Reference characters “S” and “P” shown in FIG. 2 denote systematic bits and parity bits, respectively. In FIG. 2, N systematic bits S and 2N parity bits P are stored in a CB. As shown in FIG. 2, in IR using a CB, the systematic bits (S) are extracted from among encoded bits stored in the CB by performing the time puncturing, and the extracted systematic bits (S) are transmitted in the initial transmission (in the first transmission). As shown in FIG. 2, a portion of the parity bits (P) is extracted from among the encoded bits stored in the CB by performing the time puncturing, and the extracted portion of the parity bits (P) is transmitted in the first retransmission (in the second transmission). In a similar way, a portion of the parity bits (P) is extracted from among the encoded bits stored in the CB by performing the time puncturing, and the extracted portion of the parity bits (P) is transmitted in the second retransmission (in the third transmission).